This invention relates to a semiconductor optical device which are applicable for an optical communication, an optical disk device and an optical interconnection and the like and a manufacturing method thereof, and in particular, to the semiconductor optical device which has a spot-size conversion function and a manufacturing method thereof.
In a semiconductor optical device, such as, a semiconductor laser, a semiconductor optical amplifier and a semiconductor optical modulator, a spot diameter of an optical beam which is emitted from an optical waveguide is small and further, a beam divergence is large. Consequently, it is generally difficult to couple the semiconductor optical device to an optical fiber or a silica-based optical waveguide.
To this end, the semiconductor optical device is conventionally coupled to the optical fiber or the optical waveguide by the use of an optical module with a lens. However, the lens is generally expensive, and further, the position of semiconductor optical device must be adjusted with parts, such as the lens, the optical fiber and the optical waveguide at a high accuracy. This remarkably increases the price of the optical module.
In this event, if the optical spot-size is enlarged at a facet of the semiconductor optical device and further, the beam divergence becomes narrow, the semiconductor optical semiconductor device can be coupled to the optical fiber at the high efficiency without the positional adjustment due to the high accuracy by the use of the expensive lens. Consequently, it may be possible to largely reduce the price of the optical module.
From the above-mentioned reasons, various suggestions has been conventionally made about the semiconductor optical device having the spot-size conversion function.
For instance, suggestion has been made about a semiconductor optical device of the Fabry-perot laser (thereinafter, referred to as FP-LD, and called a first conventional reference) in Japanese Unexamined Publication No. Hei. 7-283490. In this FP-LD, the waveguide of the semiconductor is integrated to convert the optical spot-size. The FP-LD has a gain region and a spot-size conversion region on a semiconductor substrate. With such a structure, the spot-size is enlarged by changing the layer thickness of the optical waveguide to realize a narrow beam divergence in the above spot-size conversion region.
On the other hand, another suggestion has been made about another FP-LD (thereinafter, called a second conventional reference) in Electronics Letters August 1996, Vol.31 No.17, pp. 1439-1440. In FP-LD of the second conventional reference, the optical spot-size is enlarged at the laser output facet by the use of a lateral direction taper shape. In this event, the lateral direction taper shape is formed by etching an epitaxial layer which is flatly grown on the entire surface of the substrate without using the selective growth method.
In the first conventional reference, the spot-size conversion region (namely, an active region) must be formed within the length between 200 xcexcm and 300 xcexcm. Consequently, the device yield for each wafer is reduced. Further, the photo-lithography steps must be twice carried out to form the selective growth mask, and the mesa-etching process must be also performed to form the waveguide. As a result, the manufacturing process inevitably becomes complicated.
On the other hand, the above-mentioned problem may be solved because the lateral taper shape having the optical gain is formed by etching the semiconductor active layer in the second conventional reference. However, the semiconductor layer must be processed in a sub-micron order at the tip portion of the tapered waveguide. Consequently, it is difficult to form the waveguide at the high accuracy by the use of the dry method in addition to the wet method. As a result, it is also difficult to uniformly fabricate the taper shape and to excellently reproduce the device characteristic.
Moreover, the device characteristic including the beam divergence largely depends upon the stripe width of the active layer in the first conventional reference. Consequently, it is difficult to stably fabricate the device having the narrow beam divergence on the condition that the excellent characteristic, the reproducibility and the uniformity of the shape are kept.
Further, the process accuracy is slightly increased in the dry method as compared to the patterning due to the wet method to form the optical waveguide. However, the active layer is damaged from the side surface in this case. Moreover, it is difficult to excellently form a buried layer at the side surface of the optical waveguide layer during growing the buried layer which is carried out after patterning the optical waveguide layer. Consequently, it is also difficult fabricate the device at a high: reliability.
To avoid this problem, the wet process must be carried out to remove the damaged layer after the dry-etching process. Finally, the high process accuracy can be practically obtained.
Further, when the coupling with the optical fiber is taken into account, it is desirable that the optical spot is formed into an approximately circular shape at the output facet with the small emission angle. However, the circular spot shape is realized only by changing the layer thickness like the first conventional reference or by forming the lateral taper shape like the second conventional reference. Consequently, the design flexibility of the device parameters, such as, the active layer structure, the active layer width and the taper shape, is remarkably restricted.
It is therefore an object of this invention to provide a semiconductor optical device which has a spot-size conversion function and which is capable of emitting an optical beam having an approximately circular spot shape with a narrow beam divergence.
It is another object of this invention to provide a method which is capable of manufacturing a semiconductor optical device having a spot-size conversion function with excellent reproducibility and uniformity of a taper shape and a device characteristic.
According to this invention, a semiconductor optical device has a gain region for oscillating a laser light beam and a spot-size conversion region for converting a spot-size of the laser light beam emitted from the gain region. Further, an optical waveguide is formed by the use of a selective growth mask along the gain region and the spot-size conversion region.
With such a structure, the optical waveguide includes a waveguide taper portion and has a width and a facet. In this event, the width gradually becomes narrower towards the facet. As a result, the waveguide taper portion is tapered along a direction from the gain region towards the facet.
More specifically, both the mask width and the opening width of the selective growth mask are formed in the taper form. Further, the optical waveguide is directly formed only by the use of the selective growth without the etching process. If the optical waveguide is formed by the use of the etching process, the epitaxial growth layer having the layer thickness of between about 0.6 xcexcm and 1.5 xcexcm must be etched. It is difficult to process the semiconductor layer having the above layer thickness in the sub-micron order with the high accuracy and the excellent reproducibility.
On the other hand, when the optical waveguide is formed by the use of the selective growth, the accuracy, the reproducibility and the uniformity of the optical waveguide depends upon the accuracy, the reproducibility and the uniformity of the selective growth mask.
Consequently, the semiconductor layer can be processed in the sub-micron order at the high accuracy and the reproducibility. This is also because the selective growth mask is formed by a dielectric thin layer having the layer thickness between 0.05 xcexcm and 0.1 xcexcm, such as, a SiO2 film and a Si3N4 film, and further, the etching depth becomes about 1/10 as compared to the case of the etching process of the epitaxial layer. Namely, the dimension accuracy and the dimension variation can be also set to about 1/10 as compared to the conventional case.
Moreover, the stripe width of the optical waveguide which is formed by the selective growth becomes narrower than the opening width of the selective growth mask. Consequently, the stripe can be formed with the dimension which is less than the resolution limit determined in the photo-lithography, for example, the dimension which is less than the quarter micron, with the high accuracy and the excellent reproducibility.
Further, the mask width itself in addition to the mask opening width gradually becomes narrower towards the facet direction in the selective growth mask which determines the lateral width dimension of the semiconductor waveguide. By adopting this method, the design flexibility is remarkably enhanced. Further, the optical spot-size can be efficiently enlarged in accordance with the waveguide length of the short spot-size converter. Moreover, the design can be easily carried out to obtain the circular optical spot shape.